1. Field of the Invention
The present invention relates to a compact zoom lens system having a high zoom ratio ranging to a considerably wide field angle, and to a focusing operation thereof.
2. Description of the Prior Art
In this field of art, there has been various attempts to provide a zoom lens system of a relatively high zoom ratio ranging to a wide field angle. Prior art zoom lens systems provided through such attempts can be roughly classified into two types, the first one having a positive front group, while the second one having a negative front group. Generally, in the zoom lens system having a zoom ratio ranging to a wide field angle, the first type is desirable for making the total length of the whole lens system more compact, and for making the diameter of the lens group located behind the front group small, even if the zoom ratio becomes relatively high. However, the second type has a disadvantage in that the total length of the whole lens system and the diameter of the lens group located behind the front group becomes undesirably large, if the zoom ratio is over 2.
Here, in the zoom lens system, the front group is conventionally shiftable in the focusing operation. However, if the positive front group is moved for the focusing operation in the zoom lens system having a zoom ratio ranging to the wide field angle, the diameter of the positive front group becomes relatively large for keeping the luminance at the periphery of the image plane, and the shortest object distance to be photographed can not be sufficiently shortened. Namely, in order to design the zoom lens system having a zoom ratio ranging to a wide field angle, a sufficient compactness, and a sufficient shortest object distance to be photographed, it is generally best to apply the first type of positive front group while performing the focusing operation by moving a lens group other than the front group.
However, in general, if the focusing operation is performed by moving the lens group other than the front group, the shifting distance of the lens group movable in the focusing operation is changed in accordance with the change of the focal length of the whole lens system. It is desirable not to change the shifting distance independently of the change of the focal length of the whole lens system.
U.S. Pat. No. 4,636,040 discloses a zoom lens system of the first type having a zoom ratio ranging to the wide field angle in which the change of the shifting distance of the focusing lens group shiftable in the focusing operation due to the change of the focal length of the whole lens system is controlled within a relatively small range. However, such a prior art zoom lens system disclosed in U.S. Pat. No. 4,636,040 has an insufficiently long total length of the whole lens system.